For years, scientists have wondered why bird species have different egg shapes. Some theorized shapes may protect eggs from shattering or allow them to fit snugly in the nest. Aristotle had even (wrongly) asserted that long, pointy eggs were female while rounder eggs were male.

But no comprehensive studies had ever been conducted to test these ideas, which left Mary Stoddard and colleagues skeptical.

The eggs of a modern ostrich and an extinct elephant bird (left) contrast with a tiny, jellybean-size hummingbird egg.

Photograph by Frans Lanting, Nat Geo Image Collection

The researchers then examined whether birds that lay eggs of a certain shape have common diets, nests, and anatomy.

For instance, they analyzed the birds' hand-wing index, a formula that describes a bird’s flight style and efficiency, as well as its ability to migrate from its birth site.

The results showed that birds with the higher hand-wing index—the most efficient and thus best fliers—were the ones with the most asymmetric or elliptical eggs. (Read about the big bang in bird evolution.)

“We were shocked to see that one of the best explanations for egg shape variation was flight ability,” says Stoddard.

“This is something that has not gotten a lot of airtime in the hypotheses that are out there for egg shape variation.”

View Images

The eggs of the common murre, a type of North American seabird, are particularly pointy.

Photograph by Frans Lanting, Nat Geo Image Collection

Dinosaur Clues

This could mean that as a bird's body evolved for better flight, the shape of its egg might have adapted with it.

Stoddard doesn't know why certain shapes correlate to better flying, but she suspects it has to do with properties of the egg membrane, which are more responsible for an egg’s shape than the shell.

The maleo, for example, may have evolved to become a skillful flier, and its egg might have evolved its long, oval shape to accommodate a streamlined body built for instantaneous flight.

“It’s really useful information about nature that up until now has been patchy,” said Claire Spottiswoode, an evolutionary biologist at the University of Cambridge, who wrote a summary of the study.

“It’s also exciting because it inspires a lot of new questions too.”

View Images

The egg collection at the University of Nebraska State Museum reveals the wide range of differences between bird eggs.

Photograph by Joel Sartore, Nat Geo Image Collection

The Penguin Exception

Based on the team’s theory, one might assume that all flightless birds would have perfectly round and symmetrical eggs. Ostriches have round eggs, for instance.

Ostrich Eggs

However, penguins have pointy eggs associated with well-flying birds. Their working hypothesis is that penguins might have adapted for powerful underwater flight, Stoddard says. (Read why penguins may have stopped flying.)

“Because they’re expert swimmers, the same processes that may influence egg shape in good fliers may also be at work in good swimmers, like penguins."